In cancer, it is metastasis which ultimately proves fatal. Metastasis is a complex, multistep process by which tumor cells invade surrounding host tissue, enter the bloodstream, travel to a distant site and establish a new colony of cells. This process involves a series of attachments and detachments. A pivotal role is played by uPA, a protease when it binds to its receptor, an adhesion molecule. As a step towards inhibiting tumor cell migration by disrupting uPA-uPAR interactions, we have grown crystals of this complex.